Rhodium and iridium salts as anti-kinking agent in direct positive silver halide emulsions



United States Patent 3,445,235 RHODIUM AND IRIDIUM SALTS AS ANTI-KINK- ING AGENT IN DIRECT POSITIVE SILVER HALIDE EMULSIONS Cortland R. Burt, Rochester, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed July 15, 1965, Ser. No. 472,313 Int. Cl. G03c 1/48, 1/36 U.S. Cl. 96-95 Claims ABSTRACT OF THE DISCLOSURE Direct positive photographic silver halide emulsion layers containing about 0.00033 gram to 12.0 grams of an amine borane per mole of silver halide, and about 1.5- 325 mg. of a salt of rhodium or iridium per mole of silver nitrate used to prepare the silver halide.

This invention relates to radiation-sensitive photographic materials, and more particularly to radiation-sensitive silver halide developing-out emulsions, and elements coated with such emulsions. Still more particularly, this invention relates to radiation-sensitive silver halide developing emulsions of the direct positive type. This invention, in addition, relates to processes for producing direct positive images utilizing the above emulsions.

Developing-out photographic emulsions are classified broadly into two types, namely (1) those which when exposed and developed, produce negative images of the originals, and (2) those which when exposed and developed, produce positive images of the originals. This invention is concerned with this latter type.

Direct positive photographic elements have long been known and are highly useful in the copying and graphic arts fields for duplicating work.

Direct positive images may be produced in a variety of ways using silver halide emulsions. For example, a silver halide emulsion may be given a short overall exposure of high intensity radiation and then given a longer imagewise exposure to radiation of lower intensity. Upon development, a direct positive image will be obtained. Another method is to expose imagewise and develop and then flash expose and redevelop. A still further method is to chemically fog the silver halide grains with, for example, formaldehyde, hydrazine, sodium arsenate, silver ions and other non-sulfide fogging agents, instead of using a solarizing exposure. Upon imagewise exposure and development of such a chemically fogged element, a positive image is obtained. In direct positive photographic elements utilizing this latter method, there is usually incorporated a desensitizing compound, usually 'a colored desensitizing dye.

All of the above methods and elements leave something to be desired and suifer from such shortcomings as low reversal density, impractically long exposure times, undesirable stain from desensitizing dyes as well as undesirable contrast. Using more than one exposure or development step adds to the number of operations and to the complexity of the process. The use of formaldehyde as a chemical fogging agent is undesirable because it has a strong effect on the hardness of the gelatin colloid carrier.

In assignees patent application, Bigelow and Burt, U.S. Ser. No. 392,625, filed Aug. 27, 1964, there is disclosed and claimed a new and improved direct positive photographic element comprising a support having coated thereon a direct positive emulsion containing as a chemical fogging agent, an amine borane compound in an amount of from 0.00033 to 12 grams per mole of silver halide.

The direct positive photographic elements made as described in the above application are extremely fast as compared to direct positive elements of the prior art and suffer from none of the disadvantages of long exposure times, undesirable stain due to desensitizing dyes, as well as undesirable contrast, and the multiplicity of exposure steps and processing operations.

However, the elements of the above Bigelow and Burt application show a pronounced tendency toward desensitization marks caused by kinking. This susceptibility toward kinking, while long recognized as a problem in high speed negative and X-ray emulsion layers, has to the best of our knowledge, never been troublesome in the prior art direct positive emulsion layers. Thishas probably been because of their very low sensitivity and slow speed. In the much more eflicient system as set forth in the above application, kinking has become a problem and manifests itself by the fact that kinked areas bleach faster than non-kinked areas during exposure.

It is an object of this invention to provide high speed direct positive emulsions free of kinking desensitization marks. It is a further object of this invention to provide direct positive photographic elements having exposure requirements equal to those required by ordinary contact, silver halide emulsion coated papers. A still further object is to provide direct positive photographic elements which eliminate the need for multiple exposure steps, multiple developing steps, or the presence of desensitizing compounds for the production of direct positive images. A still further object is to provide a process of producing direct positive images having clean whites, and good sensitometric characteristics. Other objects will appear hereinafter.

These and other objects are accomplished according to this invention by incorporating an amine borane compound as a chemical fogging agent and salts of metals of Group VIII of the Periodic Arrangement of the Elements (Handbook of Chemistry and Physics, 45th edition, The Chemical Rubber 00., Back Cover) as an anti-kinking agent in a silver halide emulsion. The emulsion preferably is a gelatino-silver chloride emulsion containing from about to by weight of silver bromide and optionally may contain, in addition to the two halides, from 0 to 6% by weight of silver iodide.

The anti-kinking agents are usually added in amounts of from about 1.50 milligrams to 325 milligrams per mole of silver nitrate and preferably are added during the precipitation phase of making the emulsion. Typical Group VIII salts are rhodium chloride, rhodium nitrate, and iridium tetrachloride. The optimum concentration of a particular Group VIII metal salt can be determined by observing the density changes in a kinked area when the element is given the customary bleaching light exposure as compared to unkinked areas. The method of kinking for test purposes involves merely bending a strip of film through an angle of about 90 over a small radius of curvature before exposure and the ratio of minimum density in a kinked area to the density in the non-kinked area is determined. The Group VIII metals include iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum.

The borane fogging agents may be added to the emulsions in amounts ranging from about 0.00033 gram to 12.0 grams per mole of silver halide and preferably from about 0.013 to 1.3 grams per mole of silver halide. They may be added either as a solid or dissolved in a suitable solvent. The effectiveness of the borane compounds in fogging a silver halide emulsion is influenced by the pH of the system, the higher the pH, the greater the fogging for a given amount of borane compound. Of course, certain of the borane compounds are more effective than others when used in equivalent quantities. The fogging effect produced by the borane compounds is bleachable by light exposure prior to development. A suitable developer may be any conventional, alkaline, photographic developing solution which would be used for standard processing of the above direct positive emulsions in the absence of the amine borane compounds. Although the mechanism is not clearly understood, the light bleaching effect is believed due to solarization. Also it is not clearly understood why soluble Group VIII metal salts such as rhodium chloride and iridium tetrachloride prevent more rapid bleaching in kinked areas and still do not affect unkinked areas. While the amine borane compounds will act to chemically fog all types of emulsion, the solarizing effect appears to be more efficient in silver chlorobromide emulsrons.

Among the amine borane compounds which have been found useful as chemical fogging compounds in this invention are the following:

(A) Alkyl amine-boranes:

Typical of these amine boranes are those of the formula where R R and R are alkyl radicals of about 1 to carbon atoms hydrogen. Examples of such boranes are:

(1) Trimethylamine borane,

(6) Alamine (tricaprylamine) borane,

CsHn

CS lT-N 2 CH3 JsHH (B) Heterocyclic amine boranes such as: 1) Pyridine borane,

(2) Picoline borane and isomeric mixtures of the three isomers of picoline borane (i.e., 0c, 18, and a picoline borane),

(3) Morpholine borane,

For most eflicient fogging action, it is preferred that the chemical fogging agents of this invention be added .4 to the silver halide emulsion after it has been made, ripened, and washed to remove the excess soluble salts resulting from the precipitation of the silver halides. Conveniently, the borane compounds are added just prior to i or during the digestion or after-ripening period. The pH of the emulsion is adjusted to between 5 and 9 preferably to 8.0-8.5 and maintained at this lever during digestion. After digestion, the pH is lowered to between 5 and 7, and the conventional coating aids are added. The emulsion is then coated and dried in the manner known to those skilled in photographic manufacturing methods.

In addition to bending test strips for kinking as described above, sensitometric characteristics of the direct positive emulsions may be determined by processing test strips of the coated layers in the following manner. A test strip in each of the following examples is fixed out in a convention photographic fixer to provide a means of establishing the minimum density (D Test strips are exposed in an intensity scale sensitometer (described on page 16, Mees, The Theory of the Photographic Process, Mac Millan Company, New York, 1942) using a /2 step wedge and log exposure 5.92. The exposed strip is developed for 1 /2 minutes at 60 F. in a developer having the composition:

Water to make 1.0 liter.

The developed strip is then immersed in a conventional short stop for 5 to 10 seconds and then fixed for 3 minutes in a conventional fixer, and then washed and dried.

In evaluation of the processed strips, the minimum density (D equals the lowest density above that of the fixed-out strip mentioned above. The maximum density is the highest density above D Total scale is given in terms of log E range and is the difference between log E (exposure) at 0.01 above D and 10g E at 3.5 above D Where in the following examples D is below 3.5, total scales are not given. The speed of a typical commercial direct positive in terms of /E 10- is 10.3 and the conventional material has a D of 0.02 and a D of 3.5 and a total scale of 0.40.

The invention will now be illustrated in and by the following examples.

EXAMPLE I To acidified aqueous solutions of gelatin each containing 1 mole of patassium chloride and the quantities of rhodium chlorida of (RhCl -3H O), as set forth in the following table, there were rapidly added solutions containing 1 mole of silver nitrate. To each of the resulting mixtures there was added 0.4 mole of potassium bromide aqueous solution and the emulsions were then ripened by heating for 10 min. at 160 F. a second 0.6 mole of potassium bromide aqueous solution was added to each and the emulsions ripened for 10 min. at 160 F.

The resulting emulsions were then cooled, washed and redispersed in the manner disclosed in Moede, US. Patent 2,772,162, issued Nov. 27, 1956. The temperature of the redispersed emulsion was raised to F. and there were then added to each 87 grams of gelatin, and the pH was adjusted to 8.2 with borax. Morpholine borane was then added according to the following table and the emulsions were digested at 130 F. for 40 minutes. The pH was reduced to 7.0 with citric acid and the usual coating aids including a gelatin hardener were added and the emulsions were each coated on a photographic film support and dried in the conventional manner.

Sensitometric evaluation adduced the following data:

Kink sensitivity 1 (ratio areas to density in nonkinked areas) Milligrams RhGla.3HaO 1 mole AgNOs IOU/E X 10 X speed 1 Ideally the kink sensitivity should be equal to one.

EXAMPLE II To an acidified aqueous solution of gelatin containing 1 mole of potassium chloride and 12 milligrams of rhodium chloride (RhCl -3H O), there was added a solution containing 1 mole of silver nitrate. To the resulting mixture there was added 0.4 mole of potassium bromide in aqueous solution. The emulsion was then ripened by heating for minutes at 160 F. A second 0.6 mole of potassium bromide in aqueous solution was added to the emulsion and it was ripened for 10 minutes at 160 F. The resulting emulsion was cooled, washed and redispersed in the manner disclosed in Example I. The temperature of the redispersed emulsion was raised to 130 F. and there was added 87 grams of gelatin and the pH was adjusted to 8.2 with borax. The emulsion was divided into two portions and to one portion (A) there was added an amount of tertiary butylamine borane to provide 0.84 milligram per mole of silver nitrate and to the second portion (B) there was added an amount of trimethylamine borane to provide 3.3 milligrams per mole of silver nitrate. The pH of the emulsions was reduced to 7.0 with citric acid and the usual coating aids including a gelatin hardener were added and the emulsions were coated on film supports and dried in the conventional manner.

Sensitometric evaluation adduced the following data:

Kink sensitivity 100/13 X 10- speed Emulsion D max.

EXAMPLE III To two acidified aqueous solutions A and B of gelatin, each containing 1 mole of potassium chloride there were added to A, 6.2 milligrams of iridium tetrachloride and to B, 24 milligrams of said iridium salt. To each of these solutions there was added rapidly a solution containing 1 mole of silver nitrate and then 0.4 mole of potassium bromide in aqueous solution. The emulsions were then ripened by heating for 10 minutes at 160 F. A second 0.6 mole of potassium bromide aqueous solution was added to each and the emulsions further ripened for 10 minutes at 160 F. The resulting emulsions were then treated as described from this stage in Example I including the addition of 1.7 milligrams of morpholine borane with results as indicated by the following sensitometric data:

Milligrams of iridium tetrachloride/mole AgNOa max- Dmin.

As indicated above in Example I, 12 milligrams of rhodium chloride appear to be the optimum for emulsions containing morpholine borane as the chemical fogging agent. Iridium salts appear to work well over a wide range of quantities.

From emulsion-coated elements of the preferred composition of the above examples, good duplicates can be made by contact-printing, using either carbon arc lamps or tungsten photoflood lamps as a light source. The emulsion-coated element may be used for reproducing continuous tone negatives, half-tones, line copy, engineering drawings, etc. The direct positive elements of this invention may also find use in color photography.

As is evident from the Sensitometric data when compared with the data on a typical commercial directive positive material set forth just before Example I above, the photographic elements of this invention provide direct positive images having extremely low minimum densities and high maximum densities and are extremely fast as compared to the direct positive elements of the prior art and at the same time are free of kink desensitization. The elements of this invention may be developed in any standard developing solution using standard techniques. Variations in the developing solution will have much the same effect as it would in developing non-reversal emulsions. No pre-exposure operations or auxiliary processing procedures are necessary or desirable in using the novel elements of this invention. It is also unnecessary to utilize stain producing desensitizing dyes or other desensitizing compounds in the emulsion. The boranes as chemical fogging agents are far superior to formaldehyde, the principal fogging agent of the prior art, because the borane compounds do not have any hardening effect on the gelatin layer and the undesirable kinking problem has been eliminated by the present compositions. This affords a method of providing wash-01f relief direct positives by using a hardening developer.

In place of gelatin other natural or synthetic waterpermeable organic colloid binding agents can be used. Such agents include water-permeable or waiter-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl others, and acetals containing a large number of extralinear CH CHOH- groups; hydrolyzed interpolymers of vinyl acetate and unsaturalted addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic acid ethyl esters and styrene. Suitable colloids of the last mentioned type are disclosed in U.S. Patents 2,276,322, 2,276,323, and 2,347,811. The useful polyvinyl acetals include polyvinyl .butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents include the poly-N-vinyllactams of Bolton U.S. Patent 2,495,918, the hydrophilic copolymers of N-acrylamido alkyl betaines described in Schacklett U.S. Patent 2,833,- 050 and hydrophilic cellulose ethers and esters.

The emulsions of this invention may be coated on any suitable base including paper and transparent film supports. For example, the cellulosic suports, e.g., cellulose acetate, cellulose triacetate, cellulose mixed esters, etc., may be used. Polymerized vinyl compounds, e.g., copolymerized vinyl acetate and vinyl chloride, polystyrene, and polymerized acrylates may also be mentioned. The film formed from the polyesters made according to the teachings of Alles, U.S. Patent 2,779,684 and the patents referred to in the specification of that patent. Other suitable supports are the polyethylene terephthalate/isophthalates of British Patent 766,290 and Canadian Patent 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate with propylene glycol, dime-thylene glycol, tetramethylene glycol or cyclohexane-1,4-dimethanol (hexahydro-p-xylene alcohol). The films of Bauer et al., U.S. Patent 3,059,543 may also be used. The above polyester films are particularly suitable because of their dimensional stability. The emulsions are generally coated on the supports in quantities to give a coating weight of about 50-75 milligrams of silver halide per square dicimeter of support surface area.

Many borane compounds other than the preferred boranes which are listed above can be used as fogging agents for direct positive photographic elements. Exemplary of such borane compounds would be selected borohydrides and polyhedral boranes. A practical limit is imposed upon the use of certain boranes due 110 high toxicity and possible explosive characteristics of individual boranes.

I claim:

1. A direct positive photographic emulsion comprising a water-permeable colloid binder, light-sensitive silver halide, about 0.00033 gram to 12.0 gramsof an amine borane per mole of silver halide and from about 1.5 milligrams to 325 milligrams of a salt of rhodium or iridium, the weight of said salt being calculated per mole of silver nitrate used to prepare said silver halide.

2. An emulsion as defined in claim 1 where said silver halide comprises a silver bromide-silver chloride mixture containing about 80 to 90% by weight of silver bromide.

3. An emulsion as defined in claim 1 where said amine borane compound is present in amounts of 0.013 to 1.3 grams per mole of silver halide.

4. An emulsion as defined in claim 1 where said metal salt is rhodium chloride.

5. An emulsion as defined in claim 1 where said metal salt is iridium tetrachloride.

6. In a direct positive photographic emulsion having a gelatin emulsion containing light-sensitive silver halide and an amine borane, the improvement of adding about 1.5 milligrams to 325 milligrams of a salt of rhodium or iridium, said salt weight being calculated per mole of silver nitrate used to prepare said silver halide.

7. A direct positive photographic element comprising a support and an emulsion layer having a water-permeable colloid binder, light-sensitive silver halide, about 0.00033 gram to 12.0 grams of an amine borane per mole of silver halide and from about 1.5 milligrams to 325 milligrams of a salt of rhodium or iridium, the weight of said salt being calculated per mole of silver nitrate used to prepare said silver halide.

8. A direct positive photographic element as defined in claim 7 where said silver halide comprises a silver bromide-silver chloride mixture containing about to by weight of silver bromide.

9. A direct positive photographic element as defined in claim 7 where said amine borane compound is present in amounts of 0.013 to 1.3 grams per mole of silver halide.

10. A direct positive photographic element as defined in claim 7 where said metal salt is rhodium chloride.

11. A direct positive photographic element as defined in claim 7 where said metal salt is iridium tetrachloride.

12. A direct positive photographic element as defined in claim 7 where said emulsion layer has a weight of about 50 :to 75 milligrams of silver halide per square decimeter of support surface.

13. In a process for preparing a direct positive photographic emulsion by forming silver halide from silver nitrate and a Water-soluble halide salt in a gelatin solution and adding an amine borane fogging agent, the improvement of adding, during the formation of said silver halide, about 15 milligrams to 325 milligrams of a salt of rhodium or iridium, said salt weight being calculated per mole of silver nitrate.

14. A process as defined in claim 13 where said metal salt is rhodium chloride.

15. A process as defined in claim 13 where said metal salt is iridium tetrachloride.

References Cited UNITED STATES PATENTS 9/1955 Wark 96-108 1/1968 Bigelow et al. 96-101 XR US. Cl. X.R. 9610l, 

